Analog compression of GPS C/A signal to audio bandwidth

ABSTRACT

The present invention discloses analog methods and circuits for compression of the GPS C/A signal to audio bandwidths to improve TTFF times, as well as decreasing auto correlation errors in systems that employ the methods and devices disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. § 119(e) of U.S.Provisional Patent Application No. 60/227,671, filed Aug. 24, 2000,entitled “ANALOG COMPRESSION OF GPS C/A SIGNAL TO AUDIO BANDWIDTH,” byCharles P. Norman, which application is incorporated by referenceherein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates in general to Global Positioning System(GPS) signals, and in particular to a method and apparatus for analogcompression of GPS clear/acquisition (C/A) signals to audio bandwidths.

2. Description of the Related Art

Currently, there exists a position determining system, referred to asthe NAVSTAR Global Positioning System (GPS), wherein a constellation of24 orbiting satellites transmit pseudo-random ranging signals from whichusers with appropriate equipment can obtain three-dimensional location,velocity and timing information anywhere on or near the surface of theEarth. The longitude, latitude and altitude of any point close to Earth,with respect to the center of the Earth, will be calculated bydetermining propagation times of electromagnetic signals from one ormore of the satellites to the point in question.

A signal about a single center frequency from one or more of the visiblesatellites will be received by a user terminal at a point close to Earthto measure propagation times of the electromagnetic signals transmittedby the satellites. The satellites from which the signals originate areidentified by modulating the signal transmitted from each satellite withpseudo-random coded signals. The GPS System will operate in twosimultaneous modes. In one mode, referred to as the clear/acquisition(C/A) mode, the pseudo-random noise (PN) signal is a Gold code sequencethat is repeated once every millisecond to enable the position of thereceiver responsive to the signals transmitted from one or more of thesatellites to be determined to an accuracy better than 100 meters.

E911 requires capability to determine the location of a handset. Thelocation information is required to be passed through a narrow bandcommunication channel. A compressed C/A code can be sent over anycellular communications channel, enabling location of the handset by thereceiver. Further, mobile devices require low power consumption, andwith compressed messages, lower transmitter and receiver power is used.

It can be seen, then, that there is a need in the art for techniques tocompress the C/A code. It can also be seen, then, that there is a needin the art for compressing the C/A code in order to use the C/A codewith other remote GPS receiver components. It can also be seen thatthere is a need in the art to provide the compressed C/A code in arelatively inexpensive manner. It can also be seen that there is an needin the art for low power GPS receivers.

SUMMARY OF THE INVENTION

To minimize the limitations in the prior art, and to minimize otherlimitations that will become apparent upon reading and understanding thepresent specification, the present invention discloses a method andapparatus for using analog circuitry to compress the GPS C/A signal toan audio bandwidth.

A method for compressing a Global Positioning System (GPS) signal inaccordance with the present invention comprises removing a carriercomponent of the GPS signal, matching a comb filter to the GPS signal toobtain a first output comprising filter lines, and frequency shiftingthe filter lines in the first output to produce a compressed GPS signal.

It is an object of the present invention to provide techniques tocompress the C/A code. It is another object of the present invention toprovide methods and devices to compress the C/A code in order to use theC/A code with other remote GPS receiver components. It is another objectof the present invention to provide methods and devices to compress theC/A code in a relatively inexpensive manner. It is also an object of thepresent invention to provide low power GPS receivers.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings in which like reference numbers representcorresponding parts throughout:

FIG. 1 illustrates a system in accordance with the present invention;and

FIGS. 2A-2H illustrates the operation of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

In the following description of the preferred embodiment, reference ismade to the accompanying drawings that form a part hereof, and in whichis shown by way of illustration a specific embodiment in which theinvention may be practiced. It is to be understood that otherembodiments may be utilized and structural changes may be made withoutdeparting from the scope of the present invention.

Overview

Any periodic signal has a Fourier series consisting of frequencies thatare integer multiples of the repetition frequency. The C/A code repeatsevery millisecond, thus the signal power is limited to frequencies thatare integer multiples of 1 kHz.

The code is modulated onto the L1 carrier frequency. This signal is thenreceived by the user and will have a Doppler shift. The receiver canremove the L1 carrier, leaving only the C/A code and Doppler. In orderto filter out the noise between the 1 kHz frequencies an assist signalneeds to be provided that places a satellite's Doppler at an integermultiple of 1 kHz. The periodicity of the C/A signal limits thebandwidth of this assist signal to 1 kHz.

The C/A code is modulated with telemetry data bits at a 50 Hz rate. Tomaintain the periodicity of the signal the telemetry data bits need tobe removed from the signal. Modulating the previously described assistsignal with the data bits will remove the data modulation. Thisincreases the bandwidth of the assist signal to 1100 Hz.

The C/A code repeats 20 times for every telemetry data bit. Thisrepetition places the signal power in spectral lines spaced 1 kHz apart.The signal can be compressed without any loss of signal by transformingthese 20 repetitions to the frequency domain and zeroing the frequencybins that have no signal. The remaining bins can be transformed back tothe time domain with the bandwidth reduced by a factor of 20. Thecompression is the same as the number of repetitions used. If the 50 Hztelemetry data bit transitions are removed then more than 20 repetitionsmay be used. The factor limiting the number of repetitions that can beused now becomes the user's motion. If the user motion is limited to 0.5g acceleration then the bandwidth may be reduced by more than a factorof 100 without loss of signal.

Additional methods of bandwidth compression are possible but may resultin signal loss. One method is to eliminate transmission of some of thespectral lines. Since the signal level in each line varies, it ispossible to eliminate the lines with the least power to minimize thesignal loss. Another method of bandwidth compression is to overlap theoutput bands. The portion of the output bands that overlap will haveincreased noise density, causing a reduction in signal to noise ratio.No loss occurs when each line is band limited such that the noise powerin the band around each line doesn't overlap the next line's band.

Example System

FIG. 1 illustrates a system in accordance with the present invention.

System 100 comprises a remote receiver 102 that has a radio link 104 toa base station 106. The radio link 104 can provide a common frequencyreference to both remote receiver 102 and base station 106, so thesystem 100 will not be concerned with user clock offsets. The basestation 106 can also provide an assist signal to remote receiver 102 viaradio link 104 to remove telemetry data and Doppler due to satellitemotion.

Remote receiver 102 comprises a GPS antenna 108 that is able to receivesignals from GPS satellites. The GPS antenna 108 can be the same antennaor a different antenna than that used to provide radio link 104. GPSsignals that are received at GPS antenna 108 are put through mixer 110,where they are mixed with L1 signal 112 to remove the L1 carrier. Theresultant signal 113 contains the GPS data in a spread spectrum format.Signal 113 is then mixed with the receive signal 114 at mixer 116, andthe resultant signal 118 is filtered in a 1 kHz comb filter 120.

The output 122 of comb filter 120 is mixed with outputs from frequencygenerators 124 at mixers 126. The outputs from mixers 126 are filteredthrough a bandpass filter signal combiner 128, and filtered output 130is duplexed through duplexer 132 for transmission on radio link 104 tobase station 106.

Operation of the Invention

FIGS. 2A-2H illustrate the operation of the invention.

In the remote unit 104 of the present invention, the bandwidth of signalthat remote unit 104 must transmit from remote unit 104 to base station106 is typically about 1 MHz when the signal is not compressed. Thepresent invention reduces the bandwidth of the signal that needs to betransmitted to base station 106 by compressing the GPS signals to abandwidth of less than 2 kHz.

Spectrum 112, shown in FIG. 2A, is essentially noise with embedded GPSsignals. Signal 114, shown in FIG. 2B, is used to shift comb filter 120by a fraction of a kHz to line the comb filter up with expected signalpeaks, or to adjust comb filter 120 to a new position to acquire and/ortrack a different satellite. The spectrum shown in FIG. 2C shows thespectrum after frequency shifting the signal to line up the signalspectral lines with the comb filter lines. 50 Hz data 130 is present inthe signal. FIG. 2D shows signal 122 as a series of lines, which resultsfrom mixing signals 112 and 114, where the 50 Hz data has been removedby mixing with signal 114, which has matching bi-phase data, and passingthe mixed signal through comb filter 120.

FIG. 2E shows the effect of mixing with frequency generators 124, whichare typically generated by a single frequency generator that is passedthrough a separate comb filter. There are typically up to 100 outputs ofthe comb filter to generate up to 100 specific frequencies. The spectrumof the output shows the first stage of compression as shown in FIG. 2F.These are then bandpass filtered by filter 128 to remove correlatedsignals, and the output signal 130 of filter 128 is shown in FIG. 2G.

FIG. 2H shows the compression process repeated to give a finalcompression ratio of up to 100.

Signal 130 requires a smaller bandwidth, i.e., a smaller time oftransmission, for transmission to base station 106. Because signal 130contains all of the information present in the original GPS signaltransmitted by the GPS satellite, the accuracy of any positiondetermination done by base station 106 is just as accurate as anypartial position calculation or full position calculation done by remotereceiver 102.

Referring again to FIG. 1, base station 106 receives signal 130 viaradio link 104, and performs a Fast Fourier Transform (FFT) 132. A localcode generator 134 also has an FFT performed in block 136, and mixer 138mixes the two signals 140 and 142. Resultant signal 144 then has anInverse FFT performed in block 146, and pseudorange information isdetermined in block 148. Essentially, base station 106 acts as a GPSreceiver and determines the position of remote receiver 102.

As described above, satellite Doppler 150 and telemetry bits 152 can bemixed in mixer 156 and transmitted to remote receiver 102 via radio link104. Further, base station 106 can transmit the position of remotereceiver 102 back to remote receiver 102 via radio link 104 for use byremote receiver 102 in location services, dead reckoning, E911situations, or other areas or services where remote receiver 102 wouldneed a position calculation.

The present invention reduces the complexity of the circuitry requiredat a remote receiver 102. There is no longer a “GPS receiver” located atremote receiver 102. Instead, there is an apparatus for removing the L1carrier and circuitry for sending compressed GPS signals to a basestation for calculation of a position of the remote receiver 102. Thepresent invention thus allows remote receivers 102, such as cellulartelephones, Personal Communications System (PCS) communication devices,Personal Data Assistant (PDA) devices, mobile computers, and othermobile devices to have a small, lightweight, low power addition andstill have access to GPS positioning technology. Further, since thebandwidth of the signal 130 is so small, an identification (ID) signalcan be attached to the signal 130 at the remote receiver, such as aMobile Identification Number/Electronic Serial Number (MIN/ESN), suchthat the base station 106 can determine which remote receiver 102 thebase station 106 is calculating a position for. Such data is useful forstatistical purposes, as well as a fee-for-service purpose of the basestation 106 or wireless carrier that is providing position calculationservices as described by the present invention.

The present invention envisions that an all-analog remote device, whichis compatible with the remainder of the GPS receiver and processingcircuitry, can transform the C/A signal such that the C/A signal can betransmitted over any audio bandwidth communication link. The transformedC/A signal can then be processed, either locally or remotely, to obtainthe location of the remote device. Further, the present inventionenvisions that an all-digital device can perform the same functions asdescribed in the analog system presented herein.

Although the above system has been presented as an example, othersystems are possible within the scope of the present invention that canbe optimized for one or more design variables, e.g., low power, for therequired output bandwidth, vehicle dynamics, sensitivity, or otherdesign goals.

Simulation

A digital simulation was performed by creating a sequence of sampled C/Acode plus noise. The compression was simplified by filtering andfrequency shifting of the sample sequence as follows:

void filterUpdate(complexType sampleData, complexType filterData[ ],long rotationIndex, long sampleSize) { long i; long thisIndex = 0; longrotationStep = (rotationIndex * TABLE_SIZE / sampleSize) % TABLE_SIZE;#define alpha 1.0 #define beta 1.0 for (i = 0; i < sampleSize; i++) {filterData[i] = complexSum( complexProduct(fromFloat(beta,0.0),filterData[i]), complexProduct(fromFloat(alpha, 0.0),complexRotation(sampleData, thisIndex))); thisIndex −= rotationStep; if(thisIndex < 0) thisIndex += TABLE_SIZE; } } complexTypeCompress(complexType filterData[ ], long compression, longrotationIndex, long sampleSize) { long i; long thisIndex = 0; longrotationStep = (rotationIndex * TABLE_SIZE / sampleSize) % TABLE_SIZE;complexType retval = complexZero; for (i = 0; i < sampleSize; i +=compression) { retval = complexSum(retval,complexRotation(filterData[i], thisIndex)); thisIndex += rotationStep;if (thisIndex >= TABLE_SIZE) thisIndex −= TABLE_SIZE; } retval.I / =sampleSize / compression; retval.Q / = sampleSize / compression; returnretval; } Part of the main program to illustrate the present inventionis: for (i = 0; i < TABLE_SIZE; i++) { filterUp date(signal[i],filterData, i, 1023 * COMPRESSION_FACTOR); SigComp[i] =Compress(filterData, COMPRESSION_FACTOR, i, 1023 * COMPRESSION_FACTOR);}

There are several ways to process the compressed samples. The mostefficient would be to sub sample at a sample rate that is twice thecompressed bandwidth. The simulation sample rate was kept the same forsimplicity. The compressed signal was transformed to the frequencydomain and then decompressed by shifting the frequency bins with thefollowing:

void deCompress(complexType compressFreq[ ], complexType deCompressFreq[],long compression, long sampleSize)

{ long i; long thisIndex = 0; for (i = 0; i < sampleSize; i++) { if (i %compression) deCompressFreq[i] = complexZero; else { deCompressFreq[i] =compressFreq[thisIndex]; thisIndex++; } } }

The decompressed samples are transformed back to the time domain and theoutput signal is compared to the input signal.

The simulation verifies that the input SNR is the same as the output SNRwhen all 1023 frequency lines are preserved and the compression does notoverlap noise bandwidth about each frequency.

CONCLUSION

Although the description of the present invention herein describesspecific embodiments of the present invention, the scope of the presentinvention includes other embodiments of the present invention notdescribed herein.

In summary, the present invention describes systems, methods andapparatuses for reducing or eliminating the auto-correlation orcross-correlation events that occur during weak signal conditions. Thedevices in accordance with the present invention also provide theability to correct the auto- or cross-correlation event to allow the GPSreceiver to lock onto the proper signal.

A method for compressing a Global Positioning System (GPS) signal inaccordance with the present invention comprises removing a carriercomponent of the GPS signal, matching a comb filter to the GPS signal toobtain a first output comprising filter lines, and frequency shiftingthe filter lines in the first output to produce a compressed GPS signal.

The foregoing description of the preferred embodiment of the inventionhas been presented for the purposes of illustration and description. Itis not intended to be exhaustive or to limit the invention to theprecise form disclosed. Many modifications and variations are possiblein light of the above teaching. It is intended that the scope of theinvention not be limited by this detailed description, but by the claimsappended hereto.

1. An apparatus for compressing a Global Positioning System (GPS) signalcomprising: a first mixer for removing a carrier component of the GPSsignal; a second mixer for receiving the carrier-removed GPS signal anda separately received frequency reference signal that is received at theapparatus separately from the GPS signal and outputting a resultantsignal; a comb filter, coupled to the second mixer, for filtering theresultant signal and obtaining a first output comprising filter lines;and a frequency shifter for shifting the filter lines in the firstoutput to produce a compressed GPS signal.
 2. The apparatus of claim 1,further comprising a second frequency shifter for shifting thecompressed GPS signal to produce a second compressed GPS signal.
 3. Theapparatus of claim 2, wherein the comb filter filters thecarrier-removed GPS signal that has been combined with the referencefrequency signal received from a remote location via a wirelesscommunication link, and using the reference frequency signal from theremote location to shift the carrier-removed GPS signal received at thecomb filter to an expected location of the filter lines of the firstoutput.
 4. The apparatus of claim 3, wherein the frequency shifting ofthe filter lines comprises mixing the filter lines with at least oneoutput of a frequency generator.
 5. A receiver comprising: at least oneantenna for receiving a GPS signal from GPS satellites and for sendingand receiving radio signals over a radio link to a base station, theradio signals including a frequency reference signal; a first mixer forremoving a carrier component of the GPS signal; a second mixer forreceiving the carrier-removed GPS signal and the frequency referencesignal and outputting a resultant signal; a comb filter, coupled to thesecond mixer, for filtering the resultant signal and obtaining a firstoutput comprising filter lines; and a frequency shifter for shifting thefilter lines in the first output to produce a compressed GPS signal. 6.The receiver of claim 5 where the at least one antenna includes a GPSantenna for receiving the GPS signal and a wireless communicationsantenna for communicating the radio link.
 7. The receiver of claim 5further comprising: a transmitter coupled to the at least one antenna tocommunicate the compressed GPS signal to the base station.
 8. Thereceiver of claim 5, further comprising a second frequency shifter forshifting the compressed GPS signal to produce a second compressed GPSsignal.
 9. The receiver of claim 8, wherein the comb filter filters thecarrier-removed GPS signal that have been combined with the referencefrequency signal received from a remote location via a wirelesscommunication link, and using the reference frequency signal from theremote location to shift the carrier-removed GPS signal received at thecomb filter to an expected location of the filter lines of the firstoutput.
 10. The receiver of claim 9, wherein the frequency shifting ofthe filter lines comprises mixing the filter lines with at least oneoutput of frequency generator.